Reflectance sensors offer a tool to diagnose spatially variable N needs and apply appropriate rates in a single trip through the field. Sensor-based systems are still in development, but are far enough advanced to be adopted by producers of corn and other crops. Over three years, we tested the performance of sensor-based variable N applications for corn in 38 field-scale trials. Plots were field-length strips, minimum treatment set was 1) the producer's normal N rate sidedressed and 2) sensor-based variable-rate sidedressing, and minimum number of replications was three. Over all trials, use of sensor-guided sidedressing reduced average N rate by 27 lb N/acre relative to rates currently used by these producers, but also reduced corn yield by nearly 2 bu/acre. Once technology and management costs are factored in, our results do not suggest any economic benefit to corn producers who adopt this practice. However, potential for N loss from the production system is almost surely reduced with no economic loss. This makes this practice an ideal target for conservation programs. Sensor-guided sidedressing has been approved for incentive payments under the Environmental Quality Incentives Program in Missouri for 2007. Results from 21 demonstrations planned for 2007 may also be reported.